|Basic Concepts in Environment, Agriculture and Natural Resources Management: An Information Kit (IIRR, 1993, 151 p.)|
|Freshwater and marine ecosystems|
Estuaries are invaluable ecosystems located along the coast where rivers meet the sea. The-mixing of the freshwater from rivers with the salty oceanic waters results in an average salinity lower than that of the open sea.
Together with the mixing of waters is the mixing of sediments from the rivers and from the sea. These sediments are deposited at the mouths of estuaries as mudflats. These flats can stretch along several kilometers of coast, especially where a great number of freshwater streams empty into the sea.
This estuarine-mudflat ecosystem lies near the mangrove forest ecosystem. Usually, there is a gradation from the muddy substrate covered by mangroves to the bare intertidal zone made up of sandflats and mudflats.
The main food source in the estuarine-mudflat ecosystem is the large quantity of organic material (detritus) coming from the mangrove forest ecosystem and usually deposited with the sediments. Primary consumers, either living on or burrowing in the mud, feed on these organic materials. Examples of these are barnacles, mussels, oysters, polychaetes, oligochaetes, burrowing molluscs and other zooplanktons. Attracted to these numerous and diverse populations present in estuaries and mudflats are secondary consumers such as shore birds, fish and invertebrate predators like crabs, some types of shrimps and carnivorous marine worms.
Other ecological values
· Serve as nursery and breeding grounds of commercially important fish resources.
· Stabilize shoreline.
· Aid in the moderation of water quantity and the purification of water.
Threats to the estuarine-mudflat ecosystem
· Pollution - Estuaries receive all the wastes carried from the land by rivers. The most dangerous of these are the non-biodegradable chemical wastes like heavy metals and pesticides, which can accumulate in the sediments and may enter the food web through the large number of invertebrates. Organic pollutants in great quantities from untreated sewage from human settlements and industrial discharges may deplete oxygen in - the water. Hence, pollution may result in less biodiversity due to death, impairment of reproductive capability and weakened resistance against diseases.
· Reclamation/Conversion - Estuaries are strategic locations for ports, industrial complexes, commercial establishments and human settlements. Reclamation for these purposes and conversion of the estuarinemudflat areas to aquaculture ponds, salt pens and farms will lead to a loss of an ecosystem important as a productive transition zone between rivers, land and sea.